References

Theses

The PhD Thesis from Bob Tubbs of Sept 2003 is available in electronic form both as a pdf file and as an HTML version (Click Here). Note the HTML version is fully electronically indexed but has lower resolution figures, The pdf version has the original high-resolution figure set.

The PhD Thesis from Nick Law entitled "Lucky Imaging: Diffraction Limited Imaging from the ground in the Visible" of July 2006 is available here in clickable HTML format. The PDF version is here (12 MBytes).

The Ph.D. Thesis from Tim Staley entitled "Lucky Imaging: beyond Binary Stars", updated on September 25, 2012 is available here in clickable PDF format (24 MB). The print version is available here (24 MB).

The Ph.D. thesis from Vincent Garrel (Observatoire de Paris) is available here (34 MB). It is in French but one of the most interesting parts of it has been published in PASP entitled "A High Sensitivity Lucky Imaging Algorithm: Image Synthesis Based on Fourier Amplitude Selection", 2012 PASP, 124:861–867, available here.

Acknowledgements

We are happy to acknowledge the help and support from a number of organisations including the Nordic Optical Telescope on La Palma, Canary Islands, The European Southern Observatory at La Silla, Chile and the Palomar Observatory in California. We have had significant support from Edmund Optics in the provision of an award that allowed the purchase of many high-quality optical components. We are also grateful to GrammaTech, Inc., of Ithaca, New York (http://www.grammatech.com/) for permission to use their CodeSonar code analysis software.

Original Lucky Imaging

Most people believe that D L Fried was the originator of the concept of "Lucky Imaging". however you may be interested to know that the significantly earlier work of Bob Hufnagel which was, in fact, referenced by Fried in his 1978 paper on Lucky Imaging anticipated the essence of that work. CDM has been in touch with Bob recently and managed to get scanned copies of his original papers which are otherwise extremely difficult to access as they were internal reports basically. Bob recently sent me a short CV which I include here:

Bob was born in New Jersey, and received his PhD in 1959 from Cornell University. He then joined the Perkin-Elmer Corporation in Connecticut,the developer of complex scientific and optical instruments, including the Hubble Space Telescope. Over the years he held the positions of Directorof Research and Chief Scientist. He made original contributions in a number of disciplines, but he is best known internationally for his pioneering publications about atmospheric turbulence. He is a Fellow of the Optical Society of America, and for several years, was an Associate Editor of its Journal, JOSA. In 1957 he married Elisa Schweikhard, of Buffalo, New York. Beginning with their honeymoon, they have since visited over 80 countries, often with backpacks and no reservations. Upon his retirement in 1990, He and Elisa have lived in Albuquerque, NM. They have three adult children, and two grandchildren.

Three of his original research documents that are much harder to find generally are now available here in PDF form,

Craig Mackay, Nick Law and Timothy D Stayley, "Diffraction Limited Imaging in the Visible from Large Ground-Based Telescopes: New Methods for Future Instruments and Telescopes", (2008), SPIE 7014, Marseille June 2008.(pdf version here)

7735-28, High-resolution imaging and spectroscopy in the visible from large ground-based telescopes with natural guide stars- C. D. Mackay, T. D. Staley, D. King, F. Suess and K. Weller, Univ. of Cambridge (United Kingdom). Copy available here (pdf format).
Near-diffraction limited imaging and spectroscopy in the visible on large (8-10 meter) class telescopes has proved to be beyond the capabilities of current adaptive optics technologies, even when using laser guidestars. The need for high resolution visible imaging in any part of the sky suggests that a rather different approach is needed. This paper describes the results of simulations, experiments and astronomical observations that show that a combination of low order adaptive optic correction using a 4-field curvature sensor and fast Lucky Imaging strategies with photon counting CCD camera systems should deliver 20-25 milliarcsecond resolution in the visible with reference stars as faint as 18.5 magnitude in I band on large telescopes. Such an instrument may be used to feed an integral field spectrograph efficiently using configurations that will also bedescribed.

7735-121, FASTCAM optomechanical system design and manufacture - G. Murga Llano, R. Sanquirce, IDOM (Spain); A. Oscoz, R. L. López, Instituto de Astrofísica de Canarias (Spain); R. Campo, IDOM (Spain)
FastCam is an instrument jointly developed by the Instituto de Astrofísica de Canarias (IAC) and the Universidad Politécnica de Cartagena (UPCT), designed to obtain high spatial resolution images in the optical wavelength range from ground-based telescopes (http://www.iac.es/proyecto/fastcam). The instrument is equipped with a very low noise and very fast readout speed EMCCD camera which provides short exposure images to an FPGA-based processor which performs the selection, recentering and combination of images in real-time (applying Lucky Imaging Techniques) to provide difraction limited resolution images in 1-4 m class telescopes from 500 to 850 nm. IDOM has contributed to this new state-of-the-art instrument with the design of an optomechanical system conceived to maximize the image scale stability of the system for astrometry. The combination of aluminum plates, carbon fiber (CFRP) rods and stainless steel mounts in the optical bench define an athermalized and stiff design to meet the requirements of thermal and mechanical stability.

7735-32, High-spatial resolution and high contrast in speckle imaging for the search of nearby companions in the optical regime - L. Labadie, R. Rebolo Lopez, B. Femenía, J. A. Pérez Prieto, Instituto de Astrofísica de Canarias (Spain); I. Villo Pérez, A. Díaz Sánchez, A. Pérez Garrido, Univ. Politécnica de Cartagena (Spain); S. R. Hildebrandt, V. Sanchez Bejar, A. Oscoz, R. L. López, Instituto de Astrofísica de Canarias (Spain)
In this paper, we present an original observational approach, which combines, for the first time, traditional speckle imaging with image postprocessing to obtain in the optical domain diffraction-limited images with high contrast (1e-5) within 0.5 to 2 arcseconds around a bright star. The post-processing step is based on wavelet filtering an has analogy with edge enhancement and high-pass filtering. Our I band on-sky results with the 2.5m Nordic Telescope (NOT) and the lucky imaging instrument FASTCAM show that we are able to detect L-type brown dwarf companions around a solar-type star with a contrast Delta_I~12 at 2 arcseconds and with no use of any coronographic capability, which greatly simplifies the instrumental and hardware approach. This object has been so far detected only in the near-infrared on AO-assisted 8-10 m class telescopes (Gemini, Keck). Discussing the advantage and disadvantage of the optical regime for the detection of faint intrinsic fluxes close to bright stars, we develop some perspectives for other fields, including the possible detection of the reflected light by planetarymass companions. To the best of our knowledge this is the first time that high contrast considerations are included in a speckle imaging approach.

7735-221, Data reduction strategies for lucky imaging, T. D. Staley, C. D. Mackay, D. King, F. Suess and K. Weller, Univ. of Cambridge (United Kingdom) Copy available here (pdf format)
Lucky Imaging is a proven technique for near diffraction limited imaging in the visible; however, data reduction and analysis techniques are relatively unexplored in the literature. We review the processes that create the rapidly varying, multiply stochastic PSF in short exposures, and use both simulated and real data to test and calibrate improved guide star registration methods including using multiple guide stars. Last, we present some results on the spatial variability of the PSF in reduced images over wide fields of view, and describe a PSF fitting package developed specifically for extracting science data from such images.

7736-132, SAM sees the light, A. A. Tokovinin, R. Tighe, P. Schurter, R. Cantarutti, N. S. van der Bliek, M. Martinez, E. Mondaca, W. Naudy, National Optical Astronomy Observatory (United States)
We present a progress report on the SOAR Adaptive Module, SAM, including some results of tests of the Natural Guide Star mode, such as corrected images in the visible, performance estimates and experiments with lucky imaging. We have tested methods to measure the seeing and the AO time constant from the loop data and compared the results to those of the the stand-alone site monitor. Measurements of the instrument throughput, telescope vibrations, and non-common-path aberrations are given. We report progress on the Laser Guide Star system implementation, including tests of the UV laser, test of the beam transfer optics with polarization control and the design of the laser launch telescope.

7736-145, Lucky imaging and adaptive optics on 10-m class telescopes: a real promise for diffraction limited imaging in the visible? B. Femenía Castellá, L. Labadie, Instituto de Astrofisica de Canarias (Spain); R. Rebolo Lopez, Instituto de Astrofisica de Canarias (Spain) and CSIC (Spain); J. A. Pérez Prieto, Instituto de Astrofisica de Canarias (Spain); A. Pérez Garrido, A. Díaz Sánchez, I. Villo Pérez, Univ. Politécnica de Cartagena (Spain)
Lucky imaging (LI) has been considered for a very long time as a cheap and/or competing alternative to adaptive optics (AO) systems. Very recent results in 4-5 m class telescopes indicate the potential benefits of having both techniques working together. In this paper we investigate from numerical simulations the potential benefits of extending such collaboration between both techniques to the domain of the 10-m class telescopes where already existing AO systems exist having been designed and optimized for operation in the NIR wavelength range. The combination of LI plus AO results not only on a better level of correction in the wavelength domain in the NIR, but also allows to extend the domain of AO correction to shorter wavelengths where AO is expected to deliver poor corrections. As a particular science case, we will show how the combination of AO+LI results in achieving a higher contrast ratio which is further enhanced when combined with a novel post-processing approach. This post-processing technique has achieved contrast ratio of up to 12 magnitudes in I-band at 2 arcsec when applied to traditional lucky imaging at 2-m class telescopes without AO. Hopes are high that significant gains in contrast ratio and angular resolution will result when applied to AO+LI systems in 10-class telescopes.

7736-179, Anisoplanatism across wide fields at highframe rates, T. D. Staley, C. D. Mackay, D. King, F. Suess and K. Weller, Univ. of Cambridge (United Kingdom) Copy available here (pdf format).
In this paper we investigate the variation of the isoplanatic patch size and other turbulent effects over short timescales and wide angular separations. We tested a visible band photon counting camera running with four 1k squared detectors to provide a contiguous field of view of 1000*4000 pixels. Resolution was 35-100 mas per pixel at frame rates from 20-111hz, providing data on wide angle atmospheric turbulence. We discuss the potential of such cameras to perform high resolution optical surveys using developments of standard “lucky imaging” techniques, and the implications of our results for adaptive optics systems design.

7736-222, The Subaru coronagraphic extreme AO (SCExAO) system: visible imaging mode, V. Garrel, National Astronomical Observatory of Japan/Subaru Telescope (United States) and LESIA, Observatoire de Paris (France); O. Guyon, National Astronomical Observatory of Japan/Subaru Telescope (United States); P. Baudoz, LESIA, Observatoire de Paris à Meudon (France); F. Martinache, F. Vogt, T. Yoshikawa, K. Yokochi, National Astronomical Observatory of Japan/Subaru Telescope (United States)
The Subaru Coronagraphic Extreme Adaptive Optics (SCExAO) system is an instrument designed to be inserted between the Subaru AO188 system and the infrared HiCIAO camera in order to greatly improve the contrast in the very close (<0.5”) neighbourhood of stars. Next to the infrared coronagraphic path, a visible scientific path, based on a EMCCD camera, has been implemented. Benefiting from both AO correction and new data processing techniques, it is a powerful tool for high angular resolution imaging and opens numerous new science opportunities. A factor 2 to 3 in Strehl ratio is obtained compared to the AO long exposure time: up to 25% Strehl in the 650nm wavelength, depending on the image processing algorithm used and the seeing conditions. The system is able to deliver diffraction limited images at 650 nm (17 mas FWHM). Our baseline image processing algorithm is based on the selection of the best signal for each spatial frequency. We demonstrate that this approach offers significantly better results than the classical select, shift and add approach (lucky imaging). We report on the first on-sky visible imaging results. We also describe how the SCExAO visible channel will also later host a high performance optical wavefront sensor based on a nonlinear curvature scheme.

7736-227, Frame selection techniques for the Magellan adaptive optics VisAO camera, J. R. Males, L. M. Close, D. A. Kopon, V. Gasho, K. Brutlag, The Univ. of Arizona (United States)
The Magellan AO system will begin commissioning in late 2011. Its VisAO camera will provide 20 mas FWHM images with mean Strehl ratios of ~0.2 in R band on a 6.5m telescope. Depending on seeing conditions, Strehl ratio may reach temporary peaks as high as 0.5 at these wavelengths. To take advantage of these brief periods of high performance, we plan to adopt “Lucky Imaging” style data taking and reduction techniques. As part of this effort we have developed a novel real-time frame selection technique, which will use AO system telemetry and a fast shutter to limit CCD exposure to these very brief moments of higher Strehl. Here we describe the expected benefits of our frame selection techniques in various operating modes. We also present the results of laboratory characterization of the shutter, and describe the performance of predictive algorithms used to control it.

7742-01, High-speed photon-counting CCD cameras for astronomy, C. D. Mackay, T. D. Staley, D. King, F. Suess and K. Weller, Univ. of Cambridge (United Kingdom). Copy available here (pdf format).
The design of electron multiplying CCD cameras require a very different approach from that appropriate for slow scan CCD operation. This paper describes the main problems in using electron multiplying CCDs for highspeed, photon counting applications in astronomy and how these may be substantially overcome. With careful design it is possible to operate the E2V Technologies L3CCDs at rates well in excess of that claimed by the manufacturer, and that levels of clock induced charge dramatically lower than those experienced with commercial cameras that need to operate at unity gain. Measurements of the performance of the E2V Technologies CCD201 operating at 26 MHz will be presented together with a guide to the effective reduction of clock induced charge levels. Examples of astronomical results obtained with our cameras will be presented.

7742-03, The darkest EMCCD ever, O. Daigle, Univ. de Montréal (Canada); S. Blais-Ouellette, Photon etc. Inc. (Canada); C. Carignan, Univ. de Montréal (Canada)
EMCCDs are devices capable of sub-electron read-out noise at high pixel rate, together with a high quantum efficiency (QE). However, they are plagued by an excess noise factor (ENF) which has the same effect on photometric measurement as if the QE would be halved. In order to get rid of the ENF, the photon counting (PC) operation is mandatory, with the drawback of counting only one photon per pixel per frame. The high frame rate capability of the EMCCDs comes to the rescue, at the price of increased clock induced charges (CIC), which dominates the noise budget of the EMCCD. The CIC can be greatly reduced with an appropriate clocking, which renders the PC operation of the EMCCD very efficient for faint flux photometry or spectroscopy, adaptive optics, ultrafast imaging and Lucky Imaging. This clocking is achievable with a new EMCCD controller: CCCP, the CCD Controller for Counting Photons. CIC levels measured with the controller are in the range of 0.001 - 0.002 electron/pixel/image, which is about an order of magnitude lower than what is commercially available. This new controller was integrated into an EMCCD camera and tested at the observatoire du Mont-Mégantic. The results are presented in this paper.

The highest resolution images ever taken in the visible were obtained by combining Lucky Imaging and low order adaptive optics. This paper describes a new instrument to be deployed on the WHT 4.2m and GTC 10.4 m telescopes on La Palma, with particular emphasis on the optical design and the expected system performance. A new design of low order wavefront sensor using photon counting CCD detectors and multi-plane curvature wavefront sensor will allow dramatically fainter reference stars to be used, allowing virtually full sky coverage with a natural guide star. This paper also describes a significant improvements in the efficiency of Lucky Imaging, important advances in wavefront reconstruction with curvature sensors and the results of simulations and sensitivity limits. With a 2 x 2 array of 1024 x 1024 photon counting EMCCDs, AOLI is likely to be the first of the new class of high sensitivity, near diffraction limited imaging systems giving higher resolution in the visible from the ground than hitherto been possible from space. Click Here

Electron Multiplying CCDs (EMCCDs) are used much less often than they might be because of the challenges they offer camera designers more comfortable with the design of slow-scan detector systems. However they offer an entirely new range of opportunities in astrophysical instrumentation. This paper will show some of the exciting new results obtained with these remarkable devices and talk about their potential in other areas of astrophysical application. We will then describe how they may be operated to give the very best performance at the lowest possible light levels. We will show that clock induced charge may be reduced to negligible levels and that, with care, devices may be clocked at significantly higher speeds than usually achieved. As an example of the advantages offered by these detectors we will show how a multi-detector EMCCD curvature wavefront sensor will revolutionise the sensitivity of adaptive optics instruments and been able to deliver the highest resolution images ever taken in the visible or the near infrared. Click Here.

SPIE vol 8446, 2012, "The AOLI low-order non-linear curvature wavefront sensor: a method for high sensitivity wavefront reconstruction", Jonathan Crass, Peter Aisher, Bruno Femenia, David L. King, Craig D. Mackay, Rafael Rebolo-López, Lucas Labadie, Antonio Pérez Garrido, Marc Balcells, Anastasio Díaz Sánchez, Jesús Jimenez Fuensalida, Roberto L. Lopez, Alejandro Oscoz, Jorge A. Pérez Prieto, Luis F. Rodríguez-Ramos, Isidro Villó. The Adaptive Optics Lucky Imager (AOLI) is a new instrument under development to demonstrate near diffraction limited imaging in the visible on large ground-based telescopes. We present the adaptive optics system being designed for the instrument comprising a large stroke deformable mirror, fixed component non-linear curvature wavefront sensor and photon-counting EMCCD detectors. We describe the optical design of the wavefront sensor where two photon-counting CCDs provide a total of four reference images. Simulations of the optical characteristics of the system are discussed, with their relevance to low and high order AO systems. The development and optimisation of high-speed wavefront reconstruction algorithms are presented. Finally we discuss the results of simulations to demonstrate the sensitivity of the system. Click Here.

P. L. Aisher; J. Crass; C. Mackay, "Wavefront phase retrieval with non-linear curvature sensors"
Monthly Notices of the Royal Astronomical Society 2012; doi: 10.1093/mnras/sts472. Abstract: Increasing interest in astronomical applications of non-linear curvature wavefront sensors for turbulence detection and correction makes it important to understand how best to handle the data they produce, particularly at low light levels. Algorithms for wavefront phase retrieval from a four-plane CWFS are developed and compared, with a view to their use for low-order phase compensation in instruments combining adaptive optics and Lucky Imaging. The convergence speed and quality of iterative algorithms is compared to their step-size, and techniques for phase retrieval at low photon counts are explored. Computer simulations show that at low light levels, preprocessing by convolution of the measured signal with a Gaussian function can reduce by an order of magnitude the photon flux required for accurate phase retrieval of low-order errors. This facilitates wavefront correction on large telescopes with very faint reference stars. Full article pdf: Click Here.

Abstract:"Lucky Imaging is now an established observing procedure that delivers near diffraction-limited images in the visible on ground-based telescopes up to ~2.5 m in diameter. Combined with low order adaptive optics it can deliver resolution several times better than that of the Hubble Space Telescope. Many images are taken at high speed as atmospheric turbulent effects appear static on these short timescales. The sharpest images are selected, shifted and added to give a much higher resolution than is normally possible in ground-based long exposure time observations. The method is relatively inefficient as a significant fraction of the frames are discarded because of their relatively poor quality. This paper shows that a new Lucky Imaging processing method involving selection in Fourier space can substantially improve the selection percentages. The results show that high resolution images with a large isoplanatic patch size may be obtained routinely both with conventional Lucky Imaging and with the new Lucky Fourier method. Other methods of improving the sensitivity of the method to faint reference stars are also described." A copy of the paper may be found here.